Certain agricultural tractor brake systems have separate left and right brake pedals, so that the brakes can assist in turning the tractor when operating at low off-road speeds. When such a tractor is operated on-road at higher speeds, the brake pedals must be manually locked together to insure that the left and right brake pressures are balanced. If an operator forgets to lock the pedals together, the operator may lose control of the tractor and implement when braking at higher on-road speeds, because the tractor will pull to the side with the highest brake pressure. A single brake pedal can be used to obtain equal left and right brake pressure. But, using a single brake pedal prohibits differential braking during tight slow speed headland turns. It is desired to have single pedal braking system for an ag tractor which provides differential braking at lower field speeds and which disables differential braking at transport speeds.